Add 2026 tax config: Australia (au)

[pascalwhoop]

Overview

Australia's FY2026-27 starts July 1, 2026. The current 2025 config covers FY2025-26. This issue tracks the FY2026-27 update — relevant from July 2026 onwards.

Key FY2026-27 changes to research

• Income tax bracket thresholds (Stage 3 cuts already applied in 2024-25, confirm further changes)
• Low Income Tax Offset (LITO) and Low and Middle Income Tax Offset (LMITO) status
• Medicare levy and surcharge thresholds
• Superannuation guarantee rate (currently stepping up annually toward 12%)

Acceptance criteria

• configs/au/2026/base.yaml with accurate FY2026-27 parameters
• Test vectors pass npm run test:configs
• Sources cited from ATO publications

Implementation

Use the /add-new-country skill. Start from the 2025 config. Target completion before July 1, 2026.